1. Field
Various embodiments relate to a memory system, and more particularly, a memory system including plural heterogeneous memories coupled to a common bus and having different latencies.
2. Description of the Related Art
In conventional computer systems, a system memory, a main memory, a primary memory, or an executable memory is typically implemented by the dynamic random access memory (DRAM). The DRAM-based memory consumes power even when no memory read operation or memory write operation is performed to the DRAM-based memory. This is because the DRAM-based memory should constantly recharge capacitors included therein. The DRAM-based memory is volatile, and thus data stored in the DRAM-based memory is lost upon removal of the power.
Conventional computer systems typically include multiple levels of caches to improve performance thereof. A cache is a high speed memory provided between a processor and a system memory in the computer system to perform an access operation to the system memory faster than the system memory itself in response to memory access requests provided from the processor. Such cache is typically implemented with a static random access memory (SRAM). The most frequently accessed data and instructions are stored within one of the levels of cache, thereby reducing the number of memory access transactions and improving performance.
Conventional mass storage devices, secondary storage devices or disk storage devices typically include one or more of magnetic media (e.g., hard disk drives), optical media (e.g., compact disc (CD) drive, digital versatile disc (DVD), etc.), holographic media, and mass-storage flash memory (e.g., solid state drives (SSDs), removable flash drives, etc.). These storage devices are Input/Output (I/O) devices because they are accessed by the processor through various I/O adapters that implement various I/O protocols. Portable or mobile devices (e.g., laptops, netbooks, tablet computers, personal digital assistant (PDAs), portable media players, portable gaming devices, digital cameras, mobile phones, smartphones, feature phones, etc.) may include removable mass storage devices (e.g., Embedded Multimedia Card (eMMC), Secure Digital (SD) card) that are typically coupled to the processor via low-power interconnects and I/O controllers.
A conventional computer system typically uses flash memory devices allowed only to store data and not to change the stored data in order to store persistent system information. For example, initial instructions such as the basic input and output system (BIOS) images executed by the processor to initialize key system components during the boot process are typically stored in the flash memory device. In order to speed up the BIOS execution speed, conventional processors generally cache a portion of the BIOS code during the pre-extensible firmware interface (PEI) phase of the boot process.
Conventional computing systems and devices include the system memory or the main memory, consisting of the DRAM, to store a subset of the contents of system non-volatile disk storage. The main memory reduces latency and increases bandwidth for the processor to store and retrieve memory operands from the disk storage.
The DRAM packages such as the dual in-line memory modules (DIMMs) are limited in terms of their memory density, and are also typically expensive with respect to the non-volatile memory storage. Currently, the main memory requires multiple DIMMs to increase the storage capacity thereof, which increases the cost and volume of the system. Increasing the volume of a system adversely affects the form factor of the system. For example, large DIMM memory ranks are not ideal in the mobile client space. What is needed is an efficient main memory system wherein increasing capacity does not adversely affect the form factor of the host system.